Multiple-input multiple-output (MIMO) systems use multiple antennas at both the transmitter and receiver of a wireless network to improve signal performance (e.g., spectral efficiency, link reliability, etc.) through the exploitation of spatial diversity. More specifically, MIMO offers significant increases in data throughput and link range without additional bandwidth or increased transmit power. Large scale antenna systems known as Massive MIMO (also referred to as very large MIMO, and hyper MIMO) use a very large number of service antennas at the base station (e.g., hundreds or thousands) with respect to the number of user equipment serviced (e.g., tens or hundreds) to focus the transmission and reception of signal energy into ever-smaller regions of space bringing improvements in throughput and energy efficiency. Other benefits of M-MIMO include that M-MIMO designs allow for the extensive use of inexpensive low-power components, reduced latency, simplification of the media access control (MAC) layer, and robustness to intentional jamming. Accordingly, techniques for integrating M-MIMO systems into next-generation wireless networks are desired.
Power density is a value used to describe how the transmit power in a communications signal is distributed over frequency. It is expressed in terms of power divided by a relatively small unit of bandwidth (e.g. dBW/kHz) and is usually referenced to the input of the antenna. The unit dBW, or dB-Watts, is the universally accepted way of expressing power on a log 10 scale (dBW=10 log 10 [PWatts]). The speed at which digital information flows is the data rate of the signal. In general, as the data rate of a signal increases, so does the range of frequencies occupied by that signal. Assuming total power in the signal is constant; increasing the data rate will spread power over a wider range of frequencies and decrease power density. The inverse is also true.
According to The Third Generation Partnership Project (3GPP) Technical Specification Group Radio Access Network, Requirements for Further Advancements for E-UTRA (LTE-Advanced or LTE-A), Release-10 (3GPP TR 36.913 V8.0.0 (2008 June)), LTE-Advanced networks should target a downlink (DL) peak data rate of 1 Gbps. In order to provide the improved data rates, LTE-Advanced introduces “multicarrier” which refers to the aggregation of multiple carriers to increase data rates. However, multi-carrier signals exhibit high peak-to-average-power-ratio (PAPR) and require expensive highly linear power amplifiers. Linear power amplifiers are also very power inefficient.
Known techniques of PAPR reduction include peak windowing, scaling, and clipping but such techniques induce interference and introduce distortion in an OFDM signal and require the signal to undergo filtering to reduce the interference and distortion to an acceptable level. Block coding is another technique to reduce PAPR.
Common broadcast signals for LTE/LTE-Advanced will now be described. Long Term Evolution (LTE), as defined by the Third Generation Partnership Project (3GPP) has two procedures for cell searching. One is for initial synchronization and the other for detecting neighbor cells in preparation for handover. In both cases, the User Equipment (UE) uses two common broadcast signals from neighboring cells, namely, a Primary Synchronization Signal (PSS) and a Secondary Synchronization Signal (SSS). The PSS and SSS signals are transmitted twice per ten millisecond radio frame and are fixed at the central sixty-two subcarriers of the channel. The detection of these signals allows the UE to complete time and frequency synchronization and to acquire useful system parameters such as cell identity, cyclic prefix length, and access mode (FDD/TDD).
The UE also decodes the Physical Broadcast Control Channel (PBCH) common broadcast signal from which it obtains important system information. The PBCH is transmitted using a Space Frequency Block Code (SFBC), which repeats every forty milliseconds, and carries what is termed as the Master Information Block (MIB) message. The MIB message on the PBCH is mapped onto the central seventy-two subcarriers of the channel.
The Physical Downlink Control Channel (PDCCH) common broadcast signal carries the resource assignment for UEs which are contained in a Downlink Control Information (DCI) message. The System Information Radio Network Temporary Identifier (SI-RNTI) is sent on the PDCCH and signals to all UEs in a cell where the broadcast System Information Blocks (SIBs) are found on the Physical Downlink Shared Channel (PDSCH).
The Physical Downlink Shared Channel (PDSCH) common broadcast signal is the main data bearing channel which is allocated to users on a dynamic and opportunistic basis. The PDSCH carries data in so-called Transport Blocks (TB).
Techniques to reduce peak-to-average power ratio (PAPR) should be low in complexity and cause minimal performance degradation and out-of-band radiation. Clipping is the simplest method to reduce PAPR but causes out-of-band radiation due to non-linear processing. Phase rotation techniques search the optimum set of phase factors. However, the search complexity of the optimum phase increases exponentially with the number of sub-blocks, and the phase factors in the receiver must be known.
An active constellation extension (ACE) technique can reduce PAPR by extending a constellation point toward the outside of the original constellation. Compared with the previously mentioned techniques, ACE induces no BER degradation and requires no special processing. However, it introduces a power increase and high complexity because of an iterative constellation extension process.
Accordingly, there is a need to provide common broadcast channel low PAPR signaling in MIMO systems while relaxing the power amplifier ratings.